There is a need for a protective coating composition which is curable at ambient temperature and provides corrosion, chemical, solvent, weathering, and heat resistance. Exemplary of surfaces requiring coating compositions with the above features are steel structures in chemical processing plants, oil refineries, coal fired power plants, and offshore drilling platforms; and internal surfaces of tanks of a petroleum tanker carrying crude oil or refined oil products such as fuel oil, lube oil, kerosene, gasoline, jet fuel and the like.
Thermoplastic coatings such as polyvinyl chloride polyvinyl acetate copolymers, thermosetting coatings based on epoxies and urethanes, and inorganic zinc coatings are used for such surfaces. However the thermoplastic coatings have inadequate heat and solvent resistance for application to surfaces such as power plant stacks and petroleum tanker tanks. Many thermosetting coatings have poor resistance to weathering due to degradation from ultraviolet light. In addition, thermosetting polymers have poor curing properties when applied at low temperatures.
Inorganic zinc coatings have been used successfully for providing corrosion resistance in moist and salty atmospheres. Exemplary of such coating compositions is that described by McLeod in U.S. Pat. No. 3,917,648. McLeod describes coating compositions containing finely divided metallic zinc held in a binder consisting essentially of the reaction product of orthosilicates and aliphatic polyols. McLeod notes that minor amounts of alkyl trialkoxyalkyl silanes can be used for preparing the binder.
Although inorganic coatings containing zinc such as that of McLeod provide corrosion, solvent and heat resistance, they lack resistance to a broad spectrum of chemicals, and in particular they are not satisfactory for petroleum tanker tanks. One problem found when inorganic coatings containing zinc are used in petroleum tanker tanks is that inorganic zinc in the coating can contaminate jet fuel with resultant sludge formation at high temperatures. A second problem is that inorganic zinc lacks resistance to crude or virgin naphtha which can contain a small amount of corrosive acid. And a third problem is that coatings such as that of McLeod which contain inorganic zinc in a silicate backbone structure lack resistance to attack by sulfur dioxide and carbon dioxide. Resistance to attack by sulfur dioxide and carbon dioxide is important because it is current practice to cycle petroleum tanker stack gas to tanks containing petroleum products to prevent explosion. Since the stack gas contains carbon dioxide and sulfur dioxide, both of which are corrosive to zinc coatings, the under deck area of oil tanker tanks when exposed to stack gas can fail within 12 months as compared to a normal life of 10 years or longer.
Also, inorganic zinc coatings have poor aesthetics and usually require a topcoat when used in a visible location.
Pure silicone resins and silicone resins polymerized with organic resins have been considered for use in such applications. However, although they can provide some chemical and solvent resistance, they are not suitable for coating interior surfaces of tanker tanks because heat cure of these resins is generally required, and this is too slow and too expensive to be used commercially in this type of application.
Therefore, there is a need for a coating for protecting surfaces from corrosion and attack by chemicals, solvents, weathering, and heat where the coating can be applied economically at ambient temperatures.